Determining and compensating for modulator dynamics in interferometric fiber-optic gyroscopes

ABSTRACT

Determining linear modulator dynamics in an interferometric fiber-optic gyroscope may be accomplished by applying a stimulus at a point within the gyroscope, observing a response in an output of the gyroscope, and determining, from the observed response, the linear modulator dynamics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and claims priority to and thebenefit of, U.S. patent application Ser. No. 12/018,016, filed on Jan.22, 2008, which claims priority to and the benefit of U.S. ProvisionalPatent Application No. 60/881,633, filed on Jan. 22, 2007. The entiredisclosures of these two applications are hereby incorporated herein byreference in their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.N00030-05-C-0007 awarded by the U.S. Navy.

TECHNICAL FIELD

The invention generally relates to interferometric fiber-opticgyroscopes. More particularly, the invention relates, in variousembodiments, to determining and compensating modulator dynamics in theinterferometric fiber-optic gyroscopes.

BACKGROUND

Interferometric fiber-optic gyroscopes are increasingly used in mediumto high performance inertial applications. For example, interferometricfiber-optic gyroscopes are used in inertial navigation applications,such as in military applications of a tactical nature (i.e., of shortrange, short time, and lower performance) and of a strategic nature(i.e., of long range, long time, and higher performance).Interferometric fiber-optic gyroscopes are also used in many commercialapplications. As one example, a tactical-grade interferometricfiber-optic gyroscope is used to stabilize the yellow line projected onthe ground during a televised football game to indicate the point thatmust be crossed by the offense to make a “first down.”

Interferometric fiber-optic gyroscopes typically use integrated-opticphase modulators to introduce a non-reciprocal phase shift tocounter-propagating light beams to aid in the measurement of inertialrate. These phase modulators often exhibit linear low-frequency dynamics(i.e., a complex gain that varies as a function of frequency).Conventional implementations of interferometric fiber-optic gyroscopestypically do not, however, compensate for these linear modulatordynamics. As a result, the presence of the linear modulator dynamicsoften degrades the performance of the gyroscopes and causes errors.

SUMMARY OF THE INVENTION

The present invention, in various embodiments, determines, compensates,reduces, and/or removes error terms associated with linear modulatordynamics that are introduced by a phase modulator into aninterferometric fiber-optic gyroscope that uses a closed loop scheme.Linear modulator dynamics are those dynamics introduced by the phasemodulator that do not typically contain nonlinear elements and that maybe characterized fully by determining the frequencies of a number ofpoles and zeroes.

In one embodiment, the present invention enables in-situ (i.e., with thephase modulator embedded in the interferometric fiber-optic gyroscopesystem) determination of the linear modulator dynamics. Morespecifically, the linear modulator dynamics may be determined throughobservation of the gyroscope's response to a signal, such as a phasemodulator step signal. The linear modulator dynamics may then becompensated for by determining a transfer function of the phasemodulator and programming, for example, a filter, such as a digitalpre-emphasis filter, to multiply a drive signal of the phase modulatorwith the inverse of its transfer function. In one embodiment, thiscompensation substantially reduces the deleterious effects of the linearmodulator dynamics on the performance of the interferometric fiber-opticgyroscope.

As described below, several advantages may be achieved using theapproach of the present invention, including the measurement of thelinear modulator dynamics in-situ and the compensation of the linearmodulator dynamics on a substantially continuous, periodic, or otherbasis. In one embodiment, phase modulator calibration is performedsubstantially continuously while the interferometric fiber-opticgyroscope measures an inertial input, which thereby improves theperformance of the gyroscope.

In general, in one aspect, the invention features a method fordetermining, in-situ, linear modulator dynamics in an interferometricfiber-optic gyroscope. A first signal is applied to a phase modulatorlocated within the interferometric fiber-optic gyroscope, a response inan output of the interferometric fiber-optic gyroscope to the firstsignal is observed, and linear modulator dynamics introduced by thephase modulator are determined from the observed response. The firstsignal applied to the phase modulator may be a step.

In general, in another aspect, the invention features a method fordetermining, in-situ, linear modulator dynamics in an interferometricfiber-optic gyroscope. A stimulus is applied at a point within theinterferometric fiber-optic gyroscope, a response in an output of theinterferometric fiber-optic gyroscope to the stimulus is observed, andlinear modulator dynamics present in the interferometric fiber-opticgyroscope are determined from the observed response.

In various embodiments, the linear modulator dynamics are determined bydetermining at least one pole and/or zero of the phase modulator and bydetermining the high frequency gain of the phase modulator. The linearmodulator dynamics may also be compensated for by pre-filtering a drivesignal before it drives the phase modulator. For example, the drivesignal may be multiplied by an inverse of a transfer function for thephase modulator. The compensation of the linear modulator dynamics mayoccur, for example, while the interferometric fiber-optic gyroscope isalso performing functions unrelated to the compensation.

In general, in yet another aspect, the invention features aninterferometric fiber-optic gyroscope. The gyroscope includes a phasemodulator and a means for pre-filtering a drive signal before it drivesthe phase modulator, thereby compensating for linear modulator dynamicspresent in the interferometric fiber-optic gyroscope.

In various embodiments, the interferometric fiber-optic gyroscope alsoincludes a means for determining a transfer function of the phasemodulator. The means for pre-filtering the drive signal may be a digitalpre-emphasis filter and may operate to multiply the drive signal by aninverse of the transfer function.

In general, in still another aspect, the invention features a method forcompensating a frequency-dependent complex gain of a phase modulator. Afirst signal is applied to a phase modulator located within aninterferometric fiber-optic gyroscope, a transfer function of the phasemodulator is determined based, at least in part, on a response in anoutput of the interferometric fiber-optic gyroscope to the first signal,and a drive signal of the phase modulator is multiplied with an inverseof the transfer function.

In general, in a further aspect, the invention features aninterferometric fiber-optic gyroscope. The gyroscope includes a phasemodulator and a means for multiplying a drive signal for the phasemodulator with an inverse of a transfer function of the phase modulatorbefore the drive signal drives the phase modulator, thereby compensatingfor linear modulator dynamics present in the interferometric fiber-opticgyroscope. In various embodiments, the interferometric fiber-opticgyroscope may further include a means for determining the transferfunction of the phase modulator.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent and may be better understood byreferring to the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates an interferometric fiber-optic gyroscope inaccordance with one embodiment of the invention;

FIG. 2A illustrates a plurality of signals present at various points inthe interferometric fiber-optic gyroscope of FIG. 1 when linearmodulator dynamics introduced therein are not compensated for;

FIG. 2B illustrates a plurality of signals present at various points inthe interferometric fiber-optic gyroscope of FIG. 1 when linearmodulator dynamics introduced therein are compensated for; and

FIG. 3 is a flow diagram of an illustrative approach to determining andcompensating for linear modulator dynamics in an interferometricfiber-optic gyroscope in accordance with the invention.

DESCRIPTION

In various embodiments, the present invention pertains to methods fordetermining and compensating, in-situ, linear modulator dynamics presentin an interferometric fiber-optic gyroscope. In broad overview, inaccordance with one embodiment of the invention, a stimulus is appliedat a point within an interferometric fiber-optic gyroscope, such as to aphase modulator of the gyroscope. A response to the stimulus is thenobserved in an output of the interferometric fiber-optic gyroscope, andthe linear modulator dynamics determined from the observed response. Themodulator dynamics may then be compensated for by, for example,multiplying a drive signal for the phase modulator with an inverse of atransfer function for the phase modulator.

FIG. 1 depicts one embodiment of an interferometric fiber-opticgyroscope 100, the performance of which may be improved using theapproach of the present invention. Among other elements, the exemplaryinterferometric fiber-optic gyroscope 100 includes an optical source110, a power splitter/coupler 112, a phase modulator (or integratedoptical circuit) 114, a fiber coil 116, an optical detector 118, phasedetection electronics 120, a baseband serrodyne generator 121, output122, a filter 124, which may be, for example, a digital pre-emphasisfilter, carrier bias modulation electronics 126, and a system processor128.

In general overview of the operation of the interferometric fiber-opticgyroscope 100 and of the propagation of light therethrough, the opticalsource 110 emanates, in one embodiment, a wavepacket that travels down asingle-mode fiber 130 acting as a mode filter. The powersplitter/coupler 112 then divides the wavepacket. In one embodiment,approximately 50% of the wavepacket is sent along the single-mode fiber130 to the phase modulator 114, with a second, remaining portion of thewavepacket being dissipated in, for example, a terminated, corelessoptical fiber 132. In one embodiment, the terminated, coreless opticalfiber prevents the second portion of the wavepacket from reflecting backinto the rest of the interferometric fiber-optic gyroscope 100.

In one embodiment, the phase modulator 114 is a Y-branch phase modulatorthat is constructed from, for example, lithium niobate (LiNbO₃)waveguides. The Y-branch phase modulator 114 includes a drive input 115for receiving, as further described below, a drive signal. In addition,the Y-branch phase modulator 114 may polarize the portion of thewavepacket it receives from the power splitter/coupler 112, and alsofurther split that portion of the wavepacket into two approximatelyequal sub-portions. One of the two sub-portions may then travel down afirst arm 134 of the phase modulator 114 and the other of the twosub-portions down a different, separate arm 136 of the phase modulator114. As depicted in FIG. 1, the first arm 134 of phase modulator 114causes the first sub-portion of the wavepacket to travel around thefiber coil 116 in a clockwise direction, while the second arm 136 of thephase modulator 114 causes the second sub-portion of the wavepacket totravel around the fiber-coil in a counter-clockwise direction.

In one embodiment, when the fiber coil 116 is stationary, eachsub-portion of the wavepacket travels the same distance in circulatingthe fiber coil 116 and thus acquires the same amount of phase. In otherwords, the two sub-portions of the wavepacket travel “reciprocal” pathsand the net phase difference between them is zero. When the fiber coil116 is rotated, however, the two sub-portions of the wavepacket mayacquire a “nonreciprocal” net phase difference due to the Sagnac effect.More specifically, the sub-portion of the wavepacket traveling in thesame direction as the rotation of the fiber coil 116 will take slightlylonger to circulate the fiber coil 116 than the sub-portion of thewavepacket traveling in a direction opposite the direction of rotationof the fiber coil 116, thereby leading to a nonreciprocal net phasedifference between the two sub-portions of the wavepacket. Thisnonreciprocal net phase difference may be increased by using multipleturns of fiber in the fiber coil 116. In particular, the nonreciprocalnet phase difference due to the Sagnac effect is given mathematicallyas:

${\Delta\;\varphi} = {\frac{8\;\pi\;{AN}}{\lambda_{0}c_{0}}\Omega}$where Δφ is the phase shift between the two sub-portions of thewavepacket, A is the area enclosed by the fiber coil 116, N is thenumber of turns in the fiber coil 116, Ω is the speed of rotation of thefiber coil 116, c₀ is the speed of light in a vacuum, and λ₀ iswavelength of light in a vacuum.

In one embodiment, as the clockwise and counter-clockwise sub-portionsof the wavepackets complete their transits through the fiber coil 116,the Y-branch phase modulator 114 recombines them and sends them backalong the single-mode fiber 130 towards the power splitter/coupler 112.As before, the power splitter/coupler 112 sends approximately 50% of therecombined wavepacket to optical detector 118, which converts thewavepacket's light into a photovoltage, while a remaining portion of therecombined wavepacket is dissipated in, for example, the optical source110.

In one embodiment, during operation of the interferometric fiber-opticgyroscope 100, carrier bias modulation electronics 126 apply asquare-wave voltage waveform to the second arm 136 of the phasemodulator 114. This bias waveform may have a maximum value of V_(π/4)volts and a minimum value of −V_(π/4) volts, where V_(π) is the voltagerequired to change the phase of light traveling through an arm of the ofthe fiber coil 116 by π radians. In one embodiment, the period of thebias waveform is 2τ, where τ is the time it takes for a wavepacket tocirculate the fiber coil 116. In one such embodiment, the carrier biasmodulation unit 126 first applies a voltage of V_(π/4) to the second arm136 of the phase modulator 114, changing the phase of, for example, theoutgoing counter-clockwise wavepacket by π/4 radians, and then, at atime τ later, applies a voltage of −V_(π/4) to the second arm 136 of thephase modulator 114, changing the phase of the incoming clockwisewavepacket by −π/4 radians. Accordingly, when the clockwise andcounter-clockwise wavepackets interfere, their phase shifts will combineto produce a net phase shift of π/2 radians. Similarly, when the carrierbias modulation unit 126 applies a voltage of −V_(π/4) to the second arm136, the phase modulator 114 shifts the phase of an outgoingcounter-clockwise wavepacket by −π/4 radians, and when the carrier biasmodulation unit 126 applies a voltage of V_(π/4) to the second arm 136,the phase modulator 114 shifts the phase of an incoming clockwisewavepacket by π/4 radians, producing a net phase shift therebetween of−π/2 radians. The optical phase between the clockwise andcounter-clockwise wavepackets may thus dither between −π/2 and π/2radians.

In one embodiment, when the fiber coil 116 is at rest, the optical phaseshift between the clockwise and counter-clockwise wavepackets willcontinuously alternate between −π/2 and π/2 radians, and each of theclockwise and counter-clockwise wavepackets will have identical lightintensity values. In such a case, the optical detector 118 outputs aconstant-level, non-zero voltage in response to the constant lightintensity inputs. As the fiber coil 16 rotates, however, the Sagnacphase shift of Δφ, described above, will cause the optical phase shiftbetween the clockwise and counter-clockwise wavepackets to alternatebetween Δφ−π/2 radians and Δφ+π/2 radians. In such a case, the opticaldetector 118 outputs a square wave (alternating between two differentvoltage levels) having the same frequency as the carrier bias modulationsignal (i.e., ½τ).

In one embodiment, the output of the optical detector 118 is sent tophase detection block 120, where it is first filtered to remove noise,and then mixed with the carrier bias modulation waveform to obtain theamplitude of the signal. This mixed signal may then amplified andfiltered to control the frequency components of the signal, prior tobeing input into a feedback loop 138.

In one embodiment, the baseband serrodyne generator 121, after receivinga signal from the phase detection block 120, outputs a serrodyne (i.e.,ramp) waveform through feedback loop 138 to drive the phase modulator114. This “closed loop” scheme of interferometric fiber-optic gyroscopeoperation keeps the optical phase between the clockwise andcounter-clockwise wavepackets output from the fiber coil 116 ditheringbetween −π/2 and π/2 radians (rather than between Δφ−π/2 radians andΔφ+π/2 radians), which maximizes sensitivity and certainty and ensuresscale factor linearity. The serrodyne signal is intended to offset theSagnac phase Δφ generated by the rotation of fiber coil 116. Themagnitude of the Sagnac phase may then be derived from the slope of theserrodyne signal, which is output at the gyroscope output 122.

Finally, as described further below, the filter 124 and the systemprocessor 128 of the interferometric fiber-optic gyroscope 100 may beused together to determine and compensate for the linear modulatordynamics introduced by the phase modulator 114 into the gyroscope 100when the gyroscope 100 is operated in the closed loop scheme. Morespecifically, the filter 124 may be any filter, such as a digitalpre-emphasis filter, capable of receiving the serrodyne signal from thebaseband serrodyne generator 121 and outputting a modified signal todrive the phase modulator 114. In one embodiment, this modified outputsignal compensates for the linear modulator dynamics introduced by thephase modulator 114. For its part, the system processor 128 may, asdescribed below, determine from the gyroscope output 122 the linearmodulator dynamics introduced by the phase modulator 114, and thenprogram the filter 124 to compensate for the linear modulator dynamics.For example, the system processor 128 may calculate a transfer functionof the phase modulator 114, and program the filter 124 to multiply theserrodyne signal output from the baseband serrodyne generator 121 withan inverse of that transfer function. The system processor 128 mayinclude and/or be implemented as any software program and/or hardwaredevice, for example as an application specific integrated circuit (ASIC)or as a field programmable gate array (FPGA), that is capable ofachieving the functionality described herein. Alternatively, theprocessor 128 may be one or more general-purpose microprocessors (e.g.,any of the PENTIUM microprocessors supplied by Intel Corp.) programmedusing any suitable programming language or languages (e.g., C++, C#,java, Visual Basic, LISP, BASIC, PERL, etc.).

FIG. 2A illustrates a plurality of signals 202, 204, 206, 208 that existin the interferometric fiber-optic gyroscope 100 when the linearmodulator dynamics introduced by the phase modulator 114 are notcompensated for (i.e., when the filter 124 is either not present or notused in the interferometric fiber-optic gyroscope 100). Morespecifically, in such a case, the serrodyne signal 204 output by thebaseband serrodyne generator 121 is used directly as a drive signal forthe phase modulator 114. The linear modulator dynamics inherent in thephase modulator 114, however, may distort an output of the phasemodulator 114 such that the actual phase modulator response signal 206differs from the desired phase modulator response signal 202. Thesedistortions in the actual phase modulator response signal 206 may causethe transient signal 208 to appear in the gyroscope output 122.

FIG. 2B illustrates, on the other hand, a plurality of signals 202, 224,226, 228 that exist in the interferometric fiber-optic gyroscope 100when the linear modulator dynamics introduced by the phase modulator 114are compensated for (i.e., when the filter 124 is present and used inthe interferometric fiber-optic gyroscope 100 as described below). Morespecifically, in such a case, the serrodyne signal 204 (see FIG. 2A)output by the baseband serrodyne generator 121 is modified by the filter124 such than a modified signal 224 drives the phase modulator 114. Asillustrated in FIG. 2B, by driving the phase modulator 114 with themodified signal 224, the transient signal 228 in the gyroscope output122 is flat (i.e., is non-existent, as the linear modulator dynamicsintroduced by the phase modulator 114 have been compensated), and theactual phase modulator 114 response signal 226 substantially matches thedesired phase modulator 114 response signal 202.

Referring now to FIG. 3, one embodiment of a method 300 for determiningand compensating for linear modulator dynamics in an interferometricfiber-optic gyroscope, for example using the components of the gyroscope100 depicted in FIG. 1, is shown. In brief overview, a stimulus isapplied to the interferometric fiber-optic gyroscope 100 at step 310, aresponse of the gyroscope 100 to the stimulus is observed at step 320,and linear modulator dynamics present in the gyroscope 100, which may beintroduced by phase modulator 114, are determined at step 330.Optionally, the linear modulator dynamics may be compensated for in step340.

In greater detail, and with reference to FIGS. 1 and 3, at step 310 astimulus or signal is applied at a point within the interferometricfiber-optic gyroscope 100. For example, in one embodiment, the stimulusis applied to the drive input 115 of the phase modulator 114. Thestimulus may be a step function applied at time t=0 and may have anamplitude of θ_(STEP). Alternatively, the stimulus may be applied atother points within the gyroscope 100. In addition, signals or stimuliother than a step function, such as, for example, an impulse signal, maybe employed. In general, any known waveform, preferably one thatcontains energy density across a frequency range of interest, may beused. As long as the waveform is known and a model exists for the phasemodulator 114, the phase modulator 114 may be characterized andcompensated as described below.

In one embodiment, the stimulus is applied to the interferometricfiber-optic gyroscope 100 prior to it being used in normal operation(i.e., prior to it being used to measure rotation). In such anembodiment, a separate device may be used to apply the stimulus.Alternatively, in another embodiment, the stimulus is applied during thenormal operation of the interferometric fiber-optic gyroscope 100 andthe gyroscope 100 is periodically calibrated and re-calibrated duringits use. For example, the stimulus may be part of the signal driving thephase modulator 114.

At step 320, the response of the interferometric fiber-optic gyroscope100 to the stimulus is observed. In one embodiment, the response, forexample the transient response 208 depicted in FIG. 2A, is observed atgyroscope output 122 by the system processor 128. Again, the response tothe stimulus may be observed by the system processor 128 prior to thegyroscope 100 being used in normal operation or while it is used innormal operation.

At step 330, the linear modulator dynamics introduced by the phasemodulator 114 are determined by the system processor 128. In oneembodiment, the system processor 128 determines the linear modulatordynamics from the response observed at step 320. Using, as an example, astep function having an amplitude of θ_(STEP) that is applied to thedrive input 115 of the phase modulator 114 at time t=0 in step 310, ithas been observed that that step function produces a small transientsignal (see, for example, the transient signal 208 depicted in FIG. 2A)in the gyroscope output 122 when linear modulator dynamics are present.In one embodiment, these linear modulator dynamics are derived by thesystem processor 128 from the characteristics of the transient signal208.

In greater detail, if the linear modulator dynamics are first-order(i.e., the phase modulator 114 frequency-domain representation includesone pole and one zero), the time-domain step response, r(t), to thisapplied step function is an exponential having the form:r(t)=Peak exp(−t/τ _(P))where Peak is the maximum value 210 (see FIG. 2A) of the time-domainstep response, t is the time, and τ_(P) is as defined below.

This response (expressed as interferometer phase) may be represented by:

${\theta_{ye}(t)} = {{{\theta_{STEP}\lbrack {\frac{\tau_{P}}{\tau_{Z}} - 1} \rbrack}\lbrack {1 - {\exp( \frac{\tau}{\tau_{P}} )}} \rbrack}{\exp( \frac{- t}{\tau_{P}} )}{u(t)}}$where θ_(ye)(t) is the relevant component of the response to the step,referred to as the Y-junction phase, u(t) denotes the unit stepfunction, and t is the time. The above equation is for a step having anamplitude of θ_(STEP) applied to the phase modulator 114 at time t=0,and a phase modulator 114 characterized by a single pole located atω_(P)=1/τ_(P) and a single zero located at ω_(Z)=1/τ_(Z). The term τdenotes the transit time for light to traverse the fiber coil 116.

In this described embodiment, the peak value 210 of the time domainresponse (including its polarity) and the area 212 (see FIG. 2A) underthe time domain response allows for direct determination of the pole andzero of the phase modulator 114. For a first-order phase modulator 114,this pole and zero completely characterize the response of the phasemodulator 114. For phase modulators 114 having higher-order dynamics,this pole and zero characterize the first-order dynamics of the phasemodulator 114.

More specifically, it has been found that:

$\tau_{P} = \frac{Area}{Peak}$and that

$\tau_{Z} = {\tau_{P}\frac{\frac{\theta_{STEP}}{Peak}\lbrack {1 - {\exp( \frac{\tau}{\tau_{P}} )}} \rbrack}{1 + {\frac{\theta_{STEP}}{Peak}\lbrack {1 - {\exp( \frac{\tau}{\tau_{P}} )}} \rbrack}}}$

In one embodiment, determining the linear modulator dynamics at step 330further includes determining the high frequency gain of the phasemodulator 114 (i.e., the voltage, V_(π, high freq), required to beapplied by the carrier bias modulation electronics 126 at highfrequencies in order to change the phase of light traveling through anarm of the of the fiber coil 116 by π radians). In one embodiment, thisvalue V_(π, high freq) is known, may be used by the carrier biasmodulation electronics 126 as described above, and is stored in thesystem processor 128. In another embodiment, the carrier bias modulationelectronics 126 or a separate device may be used to test the phasemodulator 114 to determine the high frequency gain, V_(π, high freq),and that determined value stored in the system processor 128.

Once the pole, zero, and high frequency gain of a first-order phasemodulator 114 are identified, the transfer function for the phasemodulator 114 is completely characterized. Accordingly, at step 340, thelinear modulator dynamics introduced into the interferometricfiber-optic gyroscope 100 by the phase modulator 114 may be compensatedthrough the use of the filter 124. In one embodiment, for example, thefilter 124 is a digital pre-emphasis filter that pre-filters theserrodyne drive signal output by the baseband serrodyne generator 121before it drives the phase modulator 114. In such an embodiment, thesystem processor 128 may, for example, program the filter 124 tomultiply the serrodyne drive signal by an inverse of the transferfunction for the phase modulator 114. As was previously described withreference to FIG. 2B, doing so eliminates the transient in the output122 of the interferometric fiber-optic gyroscope 100. As will beunderstood by one skilled in the art, the compensation of the linearmodulator dynamics at step 340 may occur while the gyroscope is alsoperforming functions unrelated to the compensation, such as measuringthe inertial rate of rotation.

Other approaches through which the linear modulator dynamics may bederived from the response of the gyroscope 100 to a step or otherstimulus applied to the phase modulator 114, or to a step or otherstimulus applied at another point in the gyroscope 100, exist. All suchapproaches are considered to be within the scope of the invention. Itshould also be noted that several step responses, or responses to otherstimuli, may be required to achieve the desired accuracy in themeasurement of the linear modulator dynamics. For example, for a phasemodulator 114 having second or higher-order dynamics, the first-orderdynamics and a first pole/zero pair of the phase modulator 114 may firstbe determined and compensated as described above with reference to themethod 300 of FIG. 3. The steps of the method 300 may then be repeatedto determine the second-order dynamics and a second pole/zero pair ofthe phase modulator 114, repeated again to determine the third-orderdynamics and a third pole/zero pair of the phase modulator 114, and soon until the linear modulator dynamics are either fully or sufficientlycharacterized and compensated for.

Having described certain embodiments of the invention, it will beapparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

1. An interferometric fiber-optic gyroscope, comprising: a phasemodulator; and a digital pre-emphasis filter for pre-filtering a drivesignal before it drives the phase modulator, thereby compensating forlinear modulator dynamics present in the interferometric fiber-opticgyroscope.
 2. The interferometric fiber-optic gyroscope of claim 1further comprising a means for determining a transfer function of thephase modulator.
 3. The interferometric fiber-optic gyroscope of claim2, wherein the digital pre-emphasis filter multiplies the drive signalby an inverse of the transfer function.
 4. A method for compensating afrequency-dependent complex gain of a phase modulator, comprising:applying a first signal to a phase modulator located within aninterferometric fiber-optic gyroscope; determining a transfer functionof the phase modulator based, at least in part, on a response in anoutput of the interferometric fiber-optic gyroscope to the first signal;and multiplying a drive signal of the phase modulator with an inverse ofthe transfer function.
 5. An interferometric fiber-optic gyroscope,comprising: a phase modulator; and a means for multiplying a drivesignal for the phase modulator with an inverse of a transfer function ofthe phase modulator before the drive signal drives the phase modulator,thereby compensating for linear modulator dynamics present in theinterferometric fiber-optic gyroscope.
 6. The interferometricfiber-optic gyroscope of claim 5 further comprising a means fordetermining the transfer function of the phase modulator.